It has been known for a long time that azobenzenes undergo isomerization processes under the action of light [G. C. Hartley, Nature 140, 281 (1937)]. The isomeric states and the nature of the transition reactions between the cis and trans states have been investigated in various polymers that carry azobenzenes in dispersed form or as side chains or integrated into the main chains. [C. S. Paik; H. Morawetz, Macromolecules 5, 171 (1972)].
It is also known that azobenzenes incorporated into polymers exhibit a directed orientation in an actinic light field if they are exposed to polarized light of suitable wavelength. Exposure to linearly polarized light leads for example to an excess of azobenzenes oriented perpendicularly to the direction of polarization. This may be utilized to produce a light-induced double refraction in the polymer. The orientation mechanisms of the azobenzenes have been described many times in the literature. [M. Ecih; J. H. Wendorff; B. Reck; H. Ringsdorf; Makromol. Chem. Rapid Commun. 8, 59 (1987)] [Y. Q. Shen; H. Rau, Macromol. Chem. 192, 945 (1991)].
The possibility of employing such polymers for reversible optical data storage (digital or homographic) was described for the first time by Todorov. [T. Todorov; L. Nikolova; N. Tomova, Appl. Opt. 23, 4309 (1984)]. There are many various types of materials according to the prior art for binary and/or multibit and/or volume data storage that contain azobenzenes as antennae for the incident light; see for example patent specifications EP-A 1 171 877, EP-A 1 166 187, DE-A 10 027 153, EP-A 1 166 188 and DE-A 100 271 529. Several amorphous and liquid-crystal polymers as well as oligomers have been synthesized and investigated in light exposure experiments [J. J. A. Couture; R. A. Lessard, Appl. Opt. 27, 3368 (1988)] [M. Eich; J. Wendorff, J. Opt. Soc. Am. B, 7, 1428 (1990)] [A. Natansohn; P. Rochon; J. Gosselin; S. Xie; Macromolecules 25, 2268 (1992)].
The following three factors are the basis for high light-inducible double refraction values of such polymers:    1. A high form anisotropy of the molecular side groups.
Form anisotropic components are termed mesogenes. Mesogenes are typically rod shaped, due to a part of the moleclue that is elongated and rigid. The length-width ratio measured by the van-der-Waals radii must be at least 4, preferably between 4 and 6. The form anisotropy leads to an anisotropy of the molecular polarisability. This type of molecule is described in the standard literature [H. Kelker, R. Hatz, “Handbook of Liquid Crystals”, Verlag Chemie (1980)] L. Bergmann; C. Schaefer, “Lehrbuch der Experimentalphysik”, Verlag de Gruyter, Vol. 5, “Vielteilchensysteme” (1992)].
An azo dye present in the isomeric trans state also behaves as a mesogenic molecular unit if it satisfies the said condition for form anisotropy.    2. A high number density of form anisotropic molecules, i.e. a high azobenzene content and/or a high mesogene content in the polymer.    3. A strongly anisotropic molecular orientation distribution. This is the prerequisite for the molecular anisotropies (see point 1) to be manifested macroscopically.
The degree of anisotropy may be derived from the standardised linear absorption dichroism A2, wherein A2=(2A⊥+A∥)/(3A0), in which A∥ and A⊥ denote respectively the absorption of the polymer parallel and perpendicular to the polarization direction of the actinic light and A0 denotes the absorption before the irradiation. The absorption may be determined by means of a spectrometer (e.g. Varian CARY 4G, UV-/VIS type spectrometer).
A more general description of the molecular orientation is provided by the order parameter P2=(A∥−A⊥)/(A∥+2A⊥), wherein P2=+1 and P2=−0.5 represent the limiting values for the perfect orientation of the molecular transition dipole moments parallel and perpendicular to the polarization direction of the light. P2=0 denotes the isotropic case.
In particular side chain polymers, which in addition to azobenzenes also utilize form anisotropic components as side chains, are characterized by high light-inducible double refraction since they can satisfy the three prerequisites mentioned above.
In general it is true that polymers are the more sparingly soluble the better the points 1 and 2 mentioned above are satisfied, i.e., the higher the potential is for large double refraction values. Dipole forces, geometric forces and entropy forces are responsible for this on the microscopic scale.
Many solvents, for example, alcohols that are non-poisonous or are only slightly poisonous, are therefore not suitable as solvents. Good solvents for such polymers are however in many cases poisonous, carcinogenic and/or damaging to fruit. In many cases the volatility is also too high due to their low boiling point. An example is tetrahydrofuran (THF). It is described hereinafter why such solvents are disadvantageous for the production of data storage materials, which have to satisfy particularly stringent ecological demands during manufacture.
In order to be able to use polymers as a function layer in a data storage material, these must be prepared as homogeneous films. Several casting, droplet or coating processes may be used for the production of thin films. A standard process used in large-scale production, for example of recordable compact discs (“CD-R”) and their successor formats is spin coating. In this the dyes are dissolved and the solution is dripped in an automated manner onto a rotating substrate (e.g. polycarbonate disc). After evaporation of the solvent a thin film of the recording material remains. In order to trap toxicologically problematic evaporated solvent so as to protect the environment, the production lines for data storage materials have had to be encapsulated by a complicated process, which is economically disadvantageous.
It should furthermore be noted that THF dissolves polycarbonate. An imprinted groove structure of a polycarbonate substrate would therefore be destroyed on contact with THF. In order to protect the groove structure a THF-resistant cover layer had to be applied to polycarbonate.
Reversible writing and erasing of double refraction values is a basic prerequisite for the use of a photo-addressable polymer as function layer in a rewriteable data storage material. The polymers described hitherto have the disadvantage that they do not sufficiently ensure the reversibility.
There was therefore a need for a recording material that exhibits light-inducible double refractions and that dissolves in one or more simple or modified alcohols that are non-poisonous or at least minimally poisonous. Furthermore, the recording medium should exhibit a good reversibility of the exposure dynamics.
It has surprisingly been shown that the recording materials listed in this application satisfy the aforementioned requirements.